1. Field of the Invention
The present invention relates to a color image formation apparatus using a liquid developer and a color image formation method using a liquid developer and in particular, to a color image formation apparatus using a liquid developer and a color image formation method using a liquid developer capable of realizing an accurate color expression in overlaying color images and reducing the cost by excluding the necessity of special liquid developer characteristics or high-performance squeeze characteristics.
2. Description of the Related Art
A conventional color image formation apparatus for obtaining a color image by the electronic-photographic method is, for example, disclosed in Japanese Patent Publication 11-84799 [1].
The color image formation apparatus disclosed therein is a tandem type performing an image formation without switching between color developing units and using as a developer a powder developer in which color images are overlaid on a recording medium and fixed by fixing unit for output. In this color image formation apparatus, an image formation is performed without switching the respective color developing units, which enables to form an image at a high speed.
However, in this color image formation apparatus, if powder toner particle is used for development, it is difficult to reduce the toner particle diameter less than 6 micrometers, disabling to obtain a high-quality image developing. Moreover, color images are overlaid on a recording medium whose characteristics are changed by the temperature and humidity and it is difficult to obtain a stable conveying of the recording medium, which makes it difficult to realize a high-quality color image formation. Moreover, in order to fix the color powder developer layer formed on the recording medium at a high speed by a fixing unit, the fixing unit requires an enormous energy. This substantially limits the output speed.
In order to solve this problem, for example, the published Japanese translation of PCT international publication for patent application 11-504278 [2] discloses a color image formation apparatus using a liquid developer, enabling to obtain a high-resolution image formation.
This color image formation apparatus uses as a developer a liquid developer and performs an image formation as follows. A developed image formed on a photosensitive body is transferred onto an intermediate transfer drum and the heat of the intermediate transfer drum heated while the intermediate transfer drum is rotating heats the transferred developed image into a film. This is repeated for each of the developing colors and the filmed images are transferred and fixed on a recording medium all at once. In this color image formation apparatus, use of a liquid developer enables to obtain a high-resolution image formation and color overlaying on the intermediate transfer body enables to obtain a stable color overlaying and transfer. Moreover, since a filmed color images are transferred and fixed onto the recording medium, it is possible to realize a clear color image.
However, in this color image formation apparatus, a single exposure source is used for developing each of the colors, and image transfer onto the intermediate transfer body is repeated for each of the colors. Accordingly, the output speed is lowered. In order to obtain a high-speed output, the intermediate transfer body should rotate at a high speed and the liquid developer units should be switched from one to another at a high speed. This apparatus configuration also has a high cost. Moreover, since the melting point of the toner in the liquid developer used is higher than room temperature and the intermediate transfer body is heated to a high temperature, the photosensitive body is also heated, which disables the obtaining of stable photosensitive body characteristics and shortens the usable life of the photosensitive body.
To cope with this, a color image formation apparatus as shown in FIG. 6 is suggested in xe2x80x9cFinal Program and Proceedings of ISandT""s NIP14: International Conference on Digital Printing Technologiesxe2x80x9d[3], Oct. 18, 1998. As shown in FIG. 6, this color image formation apparatus is a tandem type and uses a liquid developer. Here, firstly, a belt-shaped photosensitive body 601 is charged by a charger 602 and the charged belt-shaped photosensitive body 601 is subjected to an image exposure using a laser scan optical system 603A. One of the colors (yellow, for example) is developed by the developing unit 604A. Upon completion of the development of the color, the belt-shaped photosensitive body 601 is again charged by the next charger 602B (602C, 603D) and the image is subjected to exposure by the laser scan optical system 603B (603C, 603D). The next color (magenta, cyan, black, for example) is developed and the color image is overlaid on the belt-shaped photosensitive body 601.
The overlaid color images are transferred onto a recording medium 610 (paper, for example) by the transfer roller 605.
However, the color image formation apparatus using a liquid developer shown in FIG. 6 has following problems.
(1) Since the color images are overlaid on the photosensitive body, depending on presence/absence of a preceding toner layer developed, charging cannot be performed with an identical condition; exposure cannot be performed uniformly, i.e., the exposure light quantity applied to the photosensitive body is reduced where the toner layer is present; and the development condition cannot be uniform. This deteriorates an accurate color expression.
(2) Moreover, since a preceding image developed is brought into contact with a liquid developer for the next development, the developed image should be fixed (filmed) so that it will not be disturbed. This requires a special liquid developer characteristic and a high-performance squeeze characteristic, which costs a lot.
It is therefore an object of the present invention to provide a color image formation apparatus and a color image formation method using a liquid developer, capable of realizing a high-resolution and highly-fine color expression in overlaying the developed images and not requiring a special liquid developer characteristic or a high-performance squeeze characteristic, thereby reducing the cost.
In order to achieve the object mentioned above, the present invention provides a color image formation apparatus using a liquid developer, the apparatus comprising: a plurality of developing units, each containing a liquid developer which can be formed into a film shape; a plurality of electrostatic latent image carriers arranged for the respective developing units; an intermediate unit to which a developed image is electrostatically transferred from the plurality of electrostatic latent image carriers; and a transfer/fixation block for transferring onto a recording medium the developed image which has been formed into a film shape on the intermediate unit.
The transfer process in the transfer/fixation block does not use electrostatic force.
In the configuration mentioned above, it is preferable that the transfer from the intermediate unit to the recording medium be performed using a difference in cohesion and adhesion force.
Moreover, the transfer/fixation block preferably includes a transfer/fixation roller for heating or/and pressing the developed image on the intermediate unit. Moreover, the electrostatic latent image carrier preferably has a drum shape.
Moreover, the intermediate unit preferably has a belt shape.
Moreover, it is preferable that the developed image on the intermediate unit be formed into a film shape by heating the developed image from the opposite side of the intermediate unit not having the developed image.
Moreover, it is preferable that the developing units be in non-contact with the electrostatic latent image carriers when the liquid developer is transferred to develop the electrostatic latent image.
Moreover, it is preferable that the electrostatic latent image carriers be developed by a liquid developer having a higher solid component ratio than that of the liquid developer as initially supplied from the developing unit.
Moreover, it is preferable that the interval between the electrostatic transfers on the intermediate unit be an integral multiple of the circumferential length of the electrostatic latent image carrier, and the electrostatic latent image carriers have an identical circumferential length.
Moreover, it is preferable that each of the electrostatic latent image carriers be replaced together with the corresponding developing unit.
Moreover, the present invention provides a color image formation method using a liquid developer, the method comprising steps of: forming developed images using liquid developers having different colors and capable of being formed into a film shape on a plurality of electrostatic latent image carriers; the developed images on the plurality of electrostatic latent image carrier are successively transferred to a single image carrier belt so as to be overlaid on one another by applying an electrostatic force; increasing the solid component ratio of the liquid developer in the color developed image formed on the single image carrier belt so that the color image is formed into a film shape; and transferring and fixing the color developed image in film shape onto a recording medium.